Slab CO.sub.2 lasers were first disclosed in U.S. Pat. No. 4,719,639 which was granted to the inventor of the present invention. Slab CO.sub.2 lasers are formed from a pair of spaced apart parallel electrodes forming a channel between them, the channel having resonator mirrors positioned at opposite ends of the channel. Slab CO.sub.2 lasers also include a low pressure lasing gas (CO.sub.2), a power source of radiofrequency electromagnetic excitation for the electrodes and means for cooling the electrodes, but each of these features is well understood in the art and will not be described in any detail.
While these lasers are compact and powerful, and the slab CO.sub.2 laser represents a major advance in the art, these lasers do have limitations. The stable slab CO.sub.2 laser resonator will oscillate in a single mode up to a Fresnel number of 5. The Fresnel number for a laser is defined by N.sub.F =a.sup.2 /.lambda.d, where a is half of the aperture, .lambda. is the wavelength of the light propagating in the laser and d is the length of the laser. For lasers of a reasonable length for commercial purposes this limits the beam width and therefore the electrode width to 1 cm or less.
A particular advantage of the slab CO.sub.2 laser with a stable resonator is that the quality of the output beam is good, and this enables the beam to be focused to a diffraction limited focal spot. However, the limitation on the size of the electrode width means the power output is limited.
One solution to the problem of increasing the power of a CO.sub.2 slab laser with a stable resonator is to use an array of coupled lasers to obtain improved output power. Such a CO.sub.2 slab laser array has been described in my co-pending U.S. Pat. application No. 07/822,035 filed Jan. 14, 1992 abandoned Feb. 16, 1994. This type of slab laser array typically consists of a pair of opposing electrodes disposed to form a light guiding channel together with resonator mirrors placed at each end of the channel and means to divide the channel into a plurality of open resonators. An open resonator is a resonator that is not confined in one dimension by a waveguiding wall. The individual open resonators are coupled together by diffraction so that the array of lasers becomes coupled. The resulting array of emerging laser beams is phase related in that adjacent lasers are 180.degree. out of phase. This is known in the art as an anti-phasal array. A phase correcting plate introduced into the output beam path will phase shift the adjacent laser elements by 180.degree. so that the beams emerging from the phase shift the adjacent laser elements by 180.degree. so that the beams emerging from the phase plate are in phase; this is referred to as a co-phasal array of lasers. This type of array may be focused to an essentially single small diffraction limited spot which is a desirable feature of a CO.sub.2 laser for many applications. Because of the strong coupling between adjacent lasers in the slab laser array, as compared with for example a waveguide laser array, it is possible to couple many laser elements together and to use electrodes several centimeters wide. This results in a relatively high powered laser array.
Increasing the length and width of the electrodes used in a slab laser array will increase the laser power available. The area of electrodes in a slab laser is however limited and cannot be increased arbitrarily, due to thermal deformation of the electrodes and difficulties in matching the applied rf power to the electrodes.
A solution to the problem of using large electrodes is to use an array of smaller electrode slab CO.sub.2 lasers. It is possible to use an array of slab lasers so that their discharges are connected electrically in series. This electrical arrangement results in a relatively high voltage, low current or high impedance discharge load which is more compatible with a vacuum tube radio frequency power generator. To be useful the outputs from an array of slab lasers must be phase related, since a co-phasally related array of laser beams may be combined coherently and focused to a diffraction limited spot. Phase locking of an array of slab lasers with separate electrodes is difficult to achieve using diffraction coupling between waveguide channels. The channels are typically physically remote because of the size of the electrodes so light diffracting from one channel must traverse a large optical path length before reaching another channel.
The inventor proposes a solution to the problem of obtaining phase related output from a slab laser array by using slab lasers as a regenerative amplifier array. When used as a regenerative amplifier, the slab laser is used in a multipass mode, where the laser beam zig-zags across the channel between two mirrors placed at the ends of the electrodes. If a phase related array of laser beams is introduced into an array of slab laser amplifiers and if the amplifiers share common end mirrors then a phase related array of output laser beams may be obtained.
Since the amplified laser beam will traverse a long path length as it zig-zags between end mirrors, one might expect small path length differences caused by mirror imperfections and mechanical misalignment of electrodes and mirrors to cause phase differences in the array of amplified output beams. However, the mirrors and electrodes of a zig-zag slab waveguide amplifier may be operated with closely spaced beam paths that result in spacial interference between adjacent paths, similar to interference effects in a waveguide. The amplifier mirrors act to guide the amplified light across the width of the electrodes. Small adjustments of these mirrors will hence influence the mode of propagation across the amplifier. The mirrors may be adjusted to produce a continuous phase front across-the width of the amplifier in which the beam paths all interfere constructively. If the amplifier mirrors are plane, the phase front across the amplifier will also be plane. Curved mirrors will result in a curved phase front. The emerging beam will diffract over the output edge of the amplifier mirrors and propagate in a diffraction limited manner as is characteristic of a continuous phase laser beam.
If an amplifier array shares two mirror surfaces then the array of amplified beams will emerge with the same phase as that established by the mirror surface and the beams will hence be phase related.
For efficient operation of a laser amplifier, a long path length is desirable, but in such a case the amplifier may excite spontaneous walk off oscillation in the laser, where spontaneous emission of light is amplified across the amplifier. The resulting output beam may have poor modal quality, and an array of such beams is difficult to couple. To obtain phase related output from a phase related array of regenerative laser amplifiers it is necessary to couple a phase related array of laser beams into the amplifiers. When phase related laser beams are introduced into the amplifier the amplified light will suppress spontaneous oscillation and a diffraction limited output may be obtained. An array of regenerative amplifiers under the condition in which the mirrors are adjusted for the laser to be above the threshold for spontaneous walk off oscillation may also be referred to as an injection locked resonator array. The amplifier mirrors will hence be referred to as resonator mirrors. Each laser beam coupled to the amplifiers must have a continuous phase front or in other words be spatially coherent. Such an array may be generated by expanding the beam from a single mode laser oscillator using a cylindrical telescope and then passing this beam through an aperture which matches the spatial beam pattern required by the amplifiers. This arrangement is however inefficient because the intensity of the resulting beams is low and most of the laser light is stopped by the aperture.
A more efficient source of phase related laser beams is a slab laser oscillator array as described in U.S. Pat. application Ser. No. 07/822,035, abandoned Feb. 16, 1994. Such a laser array generates a multiplicity of phase related beams. The output from this type of laser array consists of two coincident light beams propagating at a small angle with respect to each other. This angle, .theta., is given by EQU .theta.=a/2.lambda.
where a is the width of the laser resonator in the open plane of the array, .lambda. is the laser wavelength. Coincidentally the light propagating in a waveguide may be resolved into two plane waves propagating at a small angle with respect to each other. In this case the small angle .phi. for the lowest order mode of the waveguide is given by EQU .phi.=h/2.lambda.
where h is the size of the waveguide channel. If h=a then the beam from a slab laser array will efficiently excite the first order propagation in a slab laser amplifier because the propagation angle .theta. and .phi. are equal and because the oscillator beam width a matches the amplifier channel size h.
In the far field or focus of a phased array of laser beams, the light distribution is a central lobe with symmetrically disposed side lobes. The relative intensity of the central lobe and side lobes is determined by the shape of the laser array. If the beams in the array are close and almost touching, the side lobes are very weak. If the beams of the array are spatially separated, the side lobes may be more intense than the central lobe. Since side lobes are highly undesirable for applications where a small focal spot is needed, an array of closely spaced beams is desirable. The beams from the regenerative amplifier array must be spaced by the thickness of the electrodes at the output side. It is hence desirable to expand the individual beams in the plane perpendicular to the electrodes so that the beams almost overlap. This may be done using lens arrays as are commonly used for correcting diode laser arrays. A method of avoiding such a lens array has been proposed in which the electrodes are tapered toward the output side. In this case, the beams emerging from the amplifier laser array would be more closely spaced and hence produce weaker side lobes. Such tapered electrodes would however add mechanical complexity to the amplifier array.
There is therefore proposed in one embodiment of the invention, a slab laser regenerative amplifier array including a plurality of phase related laser resonators, with each resonator having a walk-off mode of propagation of laser light from an input side of the resonator to an output side of the resonator where the exiting light diffracts around the resonator mirror. A source of a plurality of phase related light beams supplies phase related, for example co-phasal, light to each input side. The source of phase related light may be for example a source of one single mode beam followed by a telescope and plural apertures or a phase related array of laser resonators.